Angiogenesis is the process of blood vessel growth from pre-existing vasculatures. In recent years, angiogenesis has been elucidated as an important physiological phenomenon in proliferation and metastasis of various progressive solid cancers (Jain R K 2005 Science 307:58-62; Stupack et al. 2004 Curr Top Dev Biol. 64:207-238). Angiogenesis proceeds through multiple steps including, for example, 1) stimulation by vascular endothelial growth factor secreted from tumor cells; 2) disengagement of peritheliocyte or decomposition or digestion of extracellular matrix such as basal membrane; 3) migration and proliferation of vascular endothelial cells; 4) formation of tubules by the endothelial cells, formation of basal membrane, and maturation of blood vessel. In tumorous angiogenesis, the new vessels generated have the role of supplying oxygen and nutrient to tumors to accelerate their growth and serving as a route for infiltration and metastasis of tumor cells to other cells.
Age-related macular degeneration (AMD) is an angiogenesis-mediated ocular disorder in humans and is the leading cause of visual loss in individuals over age 55 (Ferris et al. 1984 Arch. Opthalmol. 102:1640-1642). There are two major clinical types of AMD: non-exudative (dry) type and exudative (wet) type. One of the pathological complications of age-related macular degeneration is choroidal angiogenesis or choroidal neovascularization (CNV). CNV is responsible for the sudden and disabling loss of central vision (Ferris et al. supra; Fine et al. 2000 N. Engl. J. Med. 342:483-492; Campochiaro et al. 2000 184:301-310).
CNV is a complex biological process and the pathogenesis of new choroidal vessel formation is not fully understood. Several factors such as inflammation (Campochiaro et al. supra; Jackson et al. 1997 FASEB J. 1997 11:457-465), ischemia (D'Amore at el. 1994 Invest Opthalmol. Vis. Sci. 35:3874-3879), and local production of angiogenic factors (Yi et al. 1997 Graefe's Arch. Clin. Exp. Opthalmol. 235:313-319; Pierce et al. 1995 PNAS 92:905-909) are thought to be important in the pathogenesis of CNV. A mouse model of laser-induced choroid neovascularization (Bora et al. 2005 J. Immunol. 174:491-497; Sakurai et al. 2003 Invest Opthamol. Vis. Sci. 44:3578-3585; Espinosa-Heidmann et al. 2003 Invest. Opthamol. Vis. Sci. 44:3586-3592) has been used to study the pathogenesis and to test potential therapeutics for AMD.
Adiponectin (APN), also called Acrp 30, adipoQ, or GBP28, is a plasma protein secreted from adipocytes and is shown to have structural similarities to C1q as well as to members of the tumor necrosis factor (TNF) superfamily. See Scherer et al. 1995 J. Biol. Chem. 270:26746-26749; Fruebis et al. 2001 PNAS 98:2005-2010; Hu et al. 1996 J. Biol. Chem. 271:10697-10703; Maeda et al. BBRC 1996 221:286-289; and Nakano et al. 1996 J. Biochem. 120: 803-812.
Adiponectin (APN) is composed of 244 amino acid residues containing a short noncollagenous N-terminal segment (about 130 amino acids) followed by a collagen-like sequence (Maeda et al. 1996 supra). Adiponectin is a homotrimer that is similar in size and overall structure to complement protein C1q, with particularly high homology (about 65-70% homology) to C1q in the C-terminal globular domain. This globular domain (about 130 amino acids) is believed to be essential for the biological activity of adiponectin. The crystal structure of adiponectin revealed additional high structural similarity between this same globular domain and TNFalpha (about 60% homology).
Several recent studies suggest that adiponectin may be a hormone that links obesity, insulin resistance, and type 2 diabetes (Maeda et al. 2001 Nat. Med. 8:731-737; Yamauchi et al. 2001 Nat. Med. 7:941-946). APN may also be an important mediator between insulin resistance and atherosclerosis and thus could be an important target for future diabetes therapy (Brakenhielm et al. 2004 Proc. Natl. Acad. Sci. USA. 101: 2476-2481). Plasma APN concentrations are significantly lower in patients with obesity, type 2 diabetes and coronary artery disease compared to control subjects (Xu et al. 2003. J. Clin. Invest 112(1): 91-100).
The vascular action of insulin to stimulate endothelial production of nitric oxide (NO), leading to vasodilation and increased blood flow, is an important component of insulin-stimulated whole body glucose utilization (Berg et al. 2002 Trends Endocrinol. Metab. 13, 84-89), as well as of coronary artery disease and macroangiopathy (Weyer et al. 2001 J Clin Endocrinol Metab. 86:1930-1935). The decreased plasma APN concentrations in diabetes may be an diagnostic indicator of macroangiopathy (Weyer et al. 2001 supra; Matsubara et al. 2002 J Clin Endocrinol Metab. 87, 2764-2769).
However, the effects of adiponectin on angiogenesis have been conflicting; for example, Shibata et al. (J. Biol. Chem. 2004 279:28670-4) and Ouchi et al. (J. Biol. Chem. 2004 279:1304-9) reported that adiponectin stimulates angiogenesis while Rose et al. (Obes. Rev. 2004 5:153-65) and Brakenhielm et al. supra have reported that adiponectin inhibits angiogenesis. Most of the experimental data reported in these studies were obtained by in vitro assays using endothelial cells derived from large vessels. Mechanisms of action of adiponectin have not been elucidated in these studies.
In order to clarify the action of adiponectin in the angiogenic process, the studies disclosed herein were undertaken, specifically, to examine the role of APN in choroidal angiogenesis. The inventors discovered that APN expression was significantly lower in the choroids of laser-induced mouse model of CNV than in the choroids of control mice. Surprisingly, it was further discovered that the administration of recombinant APN or a fragment derived therefrom to the mice having laser-induced CNV reduced the size of CNV. Therefore, the present invention provides methods of inhibiting angiogenesis, in particular choroidal angiogenesis in vivo, by administering adiponectin or a functional fragment thereof. The invention further offers a new method of treating a condition or disease involving the process of angiogenesis, particularly wet type age related macular degeneration.